U.S. patent number 5,432,452 [Application Number 08/091,916] was granted by the patent office on 1995-07-11 for device for detecting failure of battery cells by comparing the second derivative of battery voltage overtime with a preset threshold.
This patent grant is currently assigned to Merlin Gerin. Invention is credited to Jean-Noel Fiorina, Patrick Lailler.
United States Patent |
5,432,452 |
Fiorina , et al. |
July 11, 1995 |
Device for detecting failure of battery cells by comparing the
second derivative of battery voltage overtime with a preset
threshold
Abstract
Failure of one or more battery cells, for example of an inverter
battery bank, is detected by monitoring the second derivative of
the curve U(t) representative of the voltage U at the terminals of
the battery in the course of discharge. If the second derivative is
positive, this is significant of a failure.
Inventors: |
Fiorina; Jean-Noel
(Seyssinet-Pariset, FR), Lailler; Patrick (Clichy,
FR) |
Assignee: |
Merlin Gerin
(FR)
|
Family
ID: |
9432712 |
Appl.
No.: |
08/091,916 |
Filed: |
July 16, 1993 |
Foreign Application Priority Data
|
|
|
|
|
Aug 5, 1992 [FR] |
|
|
92 09819 |
|
Current U.S.
Class: |
324/427; 324/426;
320/151; 340/636.15; 340/636.1; 320/DIG.13 |
Current CPC
Class: |
G01R
31/3648 (20130101); G01R 31/3835 (20190101); G01R
31/396 (20190101); Y10S 320/13 (20130101) |
Current International
Class: |
G01R
31/36 (20060101); G01N 027/416 (); H02J
007/04 () |
Field of
Search: |
;324/426,427,429,433
;340/636 ;320/21,31,39,48 ;429/197 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Wieder; Kenneth A.
Assistant Examiner: Do; Diep
Attorney, Agent or Firm: Parkhurst, Wendel & Rossi
Claims
We claim:
1. A device for detecting failure of at least one battery cell of a
battery comprising a plurality of cells connected in series, said
device comprising:
voltage measuring means for measuring the voltage at terminals of
said battery during discharge of the battery;
computing means, connected to said voltage measuring means, for
computing a quantity representative of the second derivative of
said voltage over time;
comparing means, connected to said computing means, for comparing
said quantity with a preset, positive or nil, threshold that is
indicative of a change in sign of the second derivative; and
failure indicating means, connected to said comparing means, for
indicating said failure when said quantity is greater than said
threshold.
2. The device according to claim 1, wherein the means for measuring
the voltage comprise means for measuring the voltage at preset time
intervals and means for storing three successive values of the
voltage.
3. The device according to claim 2, wherein the means for computing
the quantity representative of the second derivative comprise means
for computing a quantity representative of the first derivative of
the voltage.
4. The device according to claim 3, wherein the means for computing
the quantity representative of the first derivative comprise means
for computing the difference between two successive voltage
values.
5. The device according to claim 4, wherein the means for computing
the quantity representative of the second derivative comprise means
for computing the difference between two successive values of the
quantity representative of the first derivative.
6. The device according to claim 1, wherein the means for computing
comprise a microprocessor.
7. A device for detecting failure of at least one battery cell of a
battery comprising a plurality of cells connected in series, said
device comprising:
voltage measuring means for measuring the voltage at terminals of
said battery during discharge of the battery;
computing means, connected to said voltage measuring means, for
computing a quantity representative of the second derivative of
said voltage over time;
comparing means, connected to said computing means, for comparing
said quantity with a preset, positive or nil, threshold;
failure indicating means, connected to said comparing means, for
indicating said failure when said quantity is greater than said
threshold;
current measuring means for measuring discharge current of the
battery;
power computing means, connected to said voltage and current
measuring means, for computing power delivered by the battery;
storing means, connected to aid power computing means, for storing
power delivered by the battery;
difference computing means, connected to said power computing means
and to aid storing means, for computing the power difference
between two successive power values;
means, connected to said difference computing means, for comparing
said power difference with a preset power threshold; and
inhibiting means for inhibiting said failure indicating means if
said power difference is greater than said power threshold.
Description
BACKGROUND OF THE INVENTION
The invention relates to a device for detecting failure of battery
cells.
Storage batteries, notably those used in Uninterruptible Power
Supplies, are generally made up of a number of cells Connected in
series. Up to now detection of failure of one or more battery cells
has only been able to be performed by monitoring the individual
voltage at the terminals of each cell. This involves measuring a
large number of voltages and requires the terminals of the
different cells to be accessible.
The object of the invention is to achieve a device for detecting
failure of one or more battery cells which is simpler and does not
present the above-mentioned drawbacks.
SUMMARY OF THE INVENTION
According to the invention, this object is achieved by the fact
that the device comprises means for measuring the voltage at
terminals of the battery, when the battery is discharging, means
for computing a quantity representative of the second derivative of
said voltage, means for comparing said quantity representative of
the second derivative with a preset, positive or nil, threshold,
and means for indicating a failure when said quantity is greater
than said threshold.
According to a preferred embodiment, the device stores three
successive values of the voltage at the terminals of the battery,
measured at preset time intervals, and computes from these three
values two successive values of a quantity representative of the
first derivative of the voltage. It then computes the quantity
representative of the second derivative by computing the difference
between the two successive values of the quantity representative of
the first derivative.
BRIEF DESCRIPTION OF THE DRAWINGS
Other advantages and features will become more clearly apparent
from the following description of an illustrative embodiment of the
invention, given as a non-restrictive example only and represented
in the accompanying drawings in which:
FIG. 1 represents, in block diagram form, an installation
comprising a device according to the invention.
FIG. 2 represents a particular embodiment of a flow chart for
implementation of the invention.
FIGS. 3 and 4 represent respectively the variation curves, versus
time, of the voltage at the terminals of the battery and of the
voltages measured at the terminals of some of its cells.
FIG. 5 illustrates an additional phase of a particular embodiment
of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 illustrates the application of a device 1 for detecting
failure of one or more battery cells to a battery 2 of a UPS. The
UPS, of conventional type, represented schematically in the figure,
is supplied by an AC voltage source 3 and comprises an AC-DC
converter 4 serially connected with a DC-AC converter, or inverter
5, and a load 6, the battery 2 being connected to the output of the
converter 4.
The device 1 comprises an electronic processing circuit 7, with
microprocessor, connected to a display device 8. The electronic
processing circuit 7 receives on input signals representative of
the voltage U at the terminals of the battery.
A failure of one or more battery cells results, when the battery is
discharging, in a break in the slope of the curve U(t)
representative of the voltage U versus time. Such a break in the
slope can be detected by computing the second derivative of the
curve U(t). In normal operation, the second derivative of the
voltage is always negative when the battery is in the course of
discharging. If the second derivative becomes positive in the
course of discharging, this is representative of failure of one or
more cells of the battery.
FIG. 2 illustrates a preferred embodiment of a flow chart which can
be implemented by the microprocessor 7. In FIG. 2, detection of
failure begins by a phase F1 in which a first measurement U3 is
made by the microprocessor 7 of the voltage U at the terminals of
the battery 2. This phase F1 is followed by a phase F2 consisting
of a second measurement U2 of the voltage U at the terminals of the
battery after a preset time interval .DELTA.t. Then the
microprocessor computes (F3) the difference D2 between U2 and U3.
The time interval .DELTA.t between two successive measurements
being preset and constant, the difference D2 is representative of
the derivative of the curve U(t).
The microprocessor then makes (F4) a third measurement U1 of the
voltage U at the terminals of the battery 2, the time interval
.DELTA.t separating the measurements of U1 and U2. Then it computes
(F5) the difference D1 between U1 and U2 and (F6) the difference D
between D1 and D2. The difference D is representative of the second
derivative of the curve U(t) constituted from the three successive
measurements U3, U2 and U1 of U made at constant preset time
intervals .DELTA.t and stored, for example in a RAM 9. Then D is
compared with a threshold, preferably zero, during a phase F7.
If D is lower than the threshold (output N of F7), the battery
cells are considered to be in good working order. The second
measurement U2 replaces (F8) the value U3 in the memory 9 of the
processing circuit 7. Then (F9) the first measurement U1 replaces
the value U2 in the memory 9 and the difference D1 replaces the
difference D2. Thus, the last two measurements are stored, along
with their difference, and the microprocessor can restart a new
cycle to determine the second derivative D at F4, by a new
measurement of U1, the time interval .DELTA.t separating two
successive measurements. As a non-restrictive example only,
.DELTA.t can be about 20 or 30s.
If this way, the second derivative is computed automatically at
each new measurement U1.
If D is greater than the threshold (output Y of F7), a failure is
indicated for example (F10) by display on the display means 8.
Indication can naturally be achieved by any other suitable means,
either visual or acoustic, locally or remotely.
In the embodiment represented in FIG. 2, the threshold
representative of a failure is zero. To avoid certain spurious
alarms it may be preferable to choose a threshold slightly greater
than zero.
It can be shown experimentally that the second derivative D is well
representative of a failure. As an example, FIG. 3 represents a
discharge curve U(t) obtained experimentally at the terminals of an
inverter battery made up of 29 cell units.
The successive values of U, measured with a time interval .DELTA.t
of 1 minute, and the successive computed values of D1 and D are as
shown below.
______________________________________ t (min) U (V) D1 D
______________________________________ 1 347.7665 2 351.5218 3.8 3
350.925 -0.6 -4.4 4 349.7962 -1.1 -0.5 5 348.2835 -1.5 -0.4 6
347.4759 -0.8 0.7 7 346.4733 -1.0 -0.2 8 345.6198 -0.9 0.1 9
344.5427 -1.1 -0.2 10 343.6921 -0.9 0.2 11 341.6315 -2.1 -1.2 12
340.0479 -1.6 0.5 13 339.2599 -0.8 0.8 14 338.4148 -0.8 -0.1 15
337.4351 -1.0 -0.1 16 336.2036 -1.2 -0.3 17 334.727 -1.5 -0.2 18
333.2822 -1.4 0.0 19 331.594 -1.7 -0.2 20 329.9797 -1.6 0.1 21
328.1715 -1.8 -0.2 22 326.8366 -1.3 0.5
______________________________________ Anomalies (D > 0) are
observed at the following times: 6 8 10 12 13 18 and 20 mins.
FIG. 4 represents the curves Ue(t) representing the variations in
the voltages at the terminals of 5 cell units during this
discharge. Significant voltage drops (Ue>10 V) can be observed
on at least one of the cell units at the following times:
5-11-12-17-19 and 20 mins. The voltages at the terminals of the
remaining cell units, not represented, did not present any
significant drop.
In the event of a large modification of the power delivered by the
battery, computation of the second derivative can lead to spurious
indications of failure. It is therefore preferable to inhibit
indication in such a case. An additional inhibition phase, as
represented in FIG. 5, can be inserted in the flow chart in FIG. 2,
for example after phase F5 or F6. This inhibition phase consists in
measuring (F11) the current Ib delivered by the battery, measured
by a current sensor 10. Then the microprocessor computes (F12) the
power delivered P1=U1Ib, and computes (F13) the difference .DELTA.p
between this power and the power measured in the previous measuring
cycle. Then .DELTA.p is compared (F14) with a preset threshold A.
If .DELTA.p is not greater than the threshold (output N of F14),
the microprocessor continues normally according to the flow chart
in FIG. 2 and goes on to phase F7. If on the other hand .DELTA.p is
greater than the threshold (output Y of F14), phases F7 and F10 are
short-circuited and the microprocessor goes directly to phase F8,
thus inhibiting any indication of failure. Phase F9 is then
completed by storing P1 in P2 for the next cycle.
The detection device described above may be combined with a
microprocessor-based device for determining the battery backup
time. Indeed, a device of this kind determines the backup time of a
battery from measurements of its voltage U and of the current Ib
delivered by the battery and from computation of the power
delivered. The existing flow chart then merely has to be completed
by a computation of the second derivative and its comparison with a
preset threshold to enable indication of failure of one or more
battery cells to be achieved.
* * * * *